A vehicle has various electronic devices (for example, an electronic control unit (ECU)) to which a battery supplies electric power. Also, the vehicle has a power supply circuit which drops an input voltage from the battery and output the dropped voltage to the electronic devices. FIG. 1 is a schematic view illustrating the circuit configuration of a power supply circuit 1a according to the related art.
The power supply circuit 1a mainly includes a switching regulator 10 and a circuit 2a including a series regulator 20a and a control circuit 30a. The power supply circuit 1a uses the switching regulator 10 to drop a voltage BATT (ideal value thereof is 14V) input from a battery 3 and outputs a predetermined voltage VIN (ideal value thereof is 6V). Then, the power supply circuit 1a uses the series regulator 20a of the circuit 2a to drop the voltage VIN, and outputs a predetermined voltage VCC (ideal value thereof is 1.2V) to a load 4 (for example, a micro computer of an ECU).
Also, each of the switching regulator 10 and the series regulator 20a regulates the voltage input thereto within a range having the input voltage as an upper limit, and outputs the regulated voltage. Therefore, if the input voltage becomes lower than a target output voltage, the output voltage also becomes lower than the target output voltage.
In other words, if the voltage BATT of the battery becomes lower than the target output voltage of each regulator, according to the drop of the voltage BATT, the voltage VIN obtained by voltage dropping of the switching regulator 10 and the voltage VCC (the voltage of the load 4) obtained by voltage dropping of the series regulator 20a also become lower than the target voltages. All of voltages to be described in this specification are DC voltages.
Next, the circuit operation of the power supply circuit 1a will be described in detail. The switching regulator 10 drops the voltage BATT input from the battery 3 to the voltage VIN, and outputs the voltage VIN. The voltage VIN output from the switching regulator 10 is input to the circuit 2a through a terminal Ta. Then, the control circuit 30a of the circuit 2a divides the input voltage VIN with resistors. A comparator 303 compares this divided voltage (hereinafter, referred to as a ‘first voltage’) with a reference voltage (hereinafter, referred to as a ‘first reference voltage’). Subsequently, according to the result of the comparison, the control circuit 30a turns on or off a switch 305 for switching the operation state of an amplifier circuit 206 of the series regulator 20a. 
The first reference voltage of the comparator 303 has a plurality of threshold values due to hysteresis, but may have one threshold value. Here, the operation of the power supply circuit 1a differs depending on whether the voltage VIN has a voltage value equal to or greater than a predetermined voltage (for example, 6V or greater) or the voltage VIN has a voltage value less than the predetermined voltage (for example, less than 6V). In other words, the output of the comparator 303 changes depending on whether the first voltage has a voltage value equal to or greater than the first reference voltage or the first voltage is less than the first reference voltage. According to the change in the output of the comparator 303, the switch 305 is turned on or off, resulting in the difference in the operation of the power supply circuit 1a. For this reason, the two cases will be separately described below.
<Case Where Voltage VIN is 6V or Greater>
In the case where the voltage VIN is 6V or greater, that is, in a case where the first voltage is equal to or greater than the first reference voltage, according to the output of the comparator 303, the switch 305 becomes an ON state. As a result, the amplifier circuit 206 of the series regulator 20a becomes an operable state.
Further, the output voltage of the series regulator 20a is divided with resistors, and this divided voltage (hereinafter, referred to as a ‘second voltage’) is input to a non-inverting input terminal of an error amplifier 204. Then, the error amplifier 204 compares the second voltage with a reference voltage (hereinafter, referred to as a ‘second reference voltage’). In a case where the second voltage has a voltage value less than the second reference voltage, the error amplifier 204 drives a PNP transistor 201 to raise the second voltage. In other words, the PNP transistor 201 becomes an ON state, a current flows between the emitter and the collector of the PNP transistor, and the potential of the voltage VCC increases. As a result, for example, even when the voltage value of the voltage BATT of the battery 3 decreases temporarily and thus the output voltage of the series regulator 20a drops, the power supply circuit 1a can output an optimal voltage for the operation of the load 4.
The error amplifier 204 operates to stop the PNP transistor 201 to drop the second voltage in the case where the second voltage has a voltage value equal to or greater than the second reference voltage. In other words, the PNP transistor 201 becomes an OFF state, any current does not flow between the emitter and the collector of the PNP transistor 201, and the potential of the voltage VCC drops. As a result, for example, even when the voltage value of the voltage BATT of the battery 3 increases temporarily and the output of the series regulator 20a rises, the power supply circuit 1a can output an optimal voltage for the operation of the load 4.
<Case Where Voltage VIN is Less Than 6V>
In the case where the voltage VIN is less than 6V, that is, in a case where the first voltage is less than the first reference voltage, according to the output of the comparator 303, the switch 305 becomes an OFF state. As a result, the amplifier circuit 206 of the series regulator stops. Therefore, a signal output from an output terminal of the error amplifier 204 is not input to the base of the PNP transistor 201, the PNP transistor 201 becomes the OFF state, and any current does not flow between the emitter and the collector of the PNP transistor 201. Like this, in the case where the voltage VIN is less than 6V, the power supply circuit 1 operates to stop the operation of the load 4. Therefore, the voltage value of the output voltage VCC of the power supply circuit 1a drops to be lower than a voltage value for making the operation of the load 4 possible. Japanese Patent Application Publication No. 2010-224825A explains a technology related to the present invention.
In the above-mentioned power supply circuit 1a, however, in a case where a vehicle transitions from an IG-ON state to an IG-OFF state, that is, in a case where the voltage value of the voltage BATT of the battery 3 drops at a predetermined rate every unit time, the voltage value of the voltage VCC to be output to the load 4 oscillates to be unstable. Here, the oscillation means that the voltage value oscillates at a constant frequency while being equal to or greater than a constant value.
Specifically, the oscillation means that the voltage value of the voltage VIN changes due to a drop of the voltage BATT and the voltage value of the voltage VCC changes according to the change of the voltage value of the voltage VIN. FIG. 2 is charts illustrating changes with time in the voltage BATT, the voltage VIN, and the voltage VCC. Hereinafter, the detailed description will be provided with reference to FIG. 2.
The upper chart of FIG. 2 is a chart illustrating the change with time in the voltage value of the voltage BATT of the battery 3 with a graph line VB. Further, the lower chart of FIG. 2 is a chart illustrating the changes with time in the voltage values of the voltage VIN and the voltage VCC with a graph line VIa and a graph line VCa, respectively. In the upper and lower charts of FIG. 2, the vertical axes represent voltage (V), and the horizontal axes represent time (μsec).
Also, in the lower chart of FIG. 2, a higher threshold value of a plurality of threshold values corresponding to the first reference voltage of the comparator 303 is referred to as a first threshold value th, and a lower threshold value of the plurality of threshold values corresponding to the first reference voltage of the comparator 303 is referred to as a second threshold value tw. In other words, when the value of the graph line VIa is equal to or greater than the first threshold value th, the switch 305 becomes the ON state, and when the value of the graph line VIa is lower than the second threshold value tw, the switch 305 becomes the OFF state. In the lower chart of FIG. 2, the first threshold value th is 3.2V, and the second threshold value tw is 3V.
First, in a case where a vehicle transitions from an IG-ON state to an IG-OFF state, as shown by the graph line VB in the upper chart of FIG. 2, the voltage value of the voltage BATT of the battery 3 decreases at a predetermined slope. At a time t1a, the graph line VB is 3V, and this represents that the voltage BATT is 3V. In this case, since the voltage BATT is less than the target output voltage value of the switching regulator 10, the value of the graph line VIa of the voltage VIN also becomes 3V. In other words, the voltage value of the voltage VIN becomes equal to or less than the second threshold value tw. As a result, the switch 305 becomes the OFF state, the amplifier circuit 206 stops, the PNP transistor 201 becomes the OFF state, and any current does not flow between the emitter and the collector of the PNP transistor 201. In other words, the series regulator 20a stops operation thereof.
Further, if the voltage value of the voltage VIN becomes equal to or less than 3V, as shown by the graph line VCa, the voltage value of the voltage VCC also decreases. As a result, the voltage VCC becomes a voltage value less than a voltage of 1.2V with which the load 4 is operable.
Here, if the series regulator 20a stops outputting, any current does not flow from the terminal Ta to a terminal Tb. Then, due to self-induced electromotive force generated in a coil 102 of the switching regulator 10 shown in FIG. 1, the voltage temporarily rises so that a current continuously flows in the same direction. As a result, despite the decrease of the voltage value of the battery 3, the value of the voltage VIN increases. Then, when the value of the voltage VIN becomes equal to or greater than the first threshold value th at a time t2a, the amplifier circuit 206 becomes operable, and the PNP transistor 201 becomes the ON state. As a result, a relatively large amount of electric charge flows between the emitter and collector of the PNP transistor 201, and the value of the output voltage VCC of the power supply circuit 1a increases at a predetermined or greater slope.
Subsequently, according to a large amount of current output from the power supply circuit 1a, the voltage value of the voltage VIN decreases at a predetermined or greater slope. Then, the value of the voltage VCC becomes 3V or less at a time t3a within a relatively short time (for example, at a frequency of 20 μsec). As a result, the amplifier circuit 206 stops operation thereof, and the PNP transistor 201 becomes the OFF state and thus any current does not flow between the emitter and the collector of the PNP transistor 201.
When the voltage value of the voltage VIN becomes 3V or less, the value of the voltage VCC also decreases. As a result, the voltage VCC becomes less than the voltage (1.2 V) for operating the load 4. In this case, since the series regulator 20a stops outputting, the value of the voltage VIN which is input from the switching regulator 10 to the circuit 2a increases due to the characteristics of the coil 102 of the switching regulator 10 or the like, as described above. Then, when the value of the voltage VIN becomes the first threshold value th or greater at a time t4a, the amplifier circuit 206 becomes operable, and the PNP transistor 201 becomes the ON state. As a result, a relatively large amount of current flows between the emitter and collector of the PNP transistor 201, and thus the voltage value of the output voltage VCC of the power supply circuit 1a increases at a predetermined or greater slope. When the vehicle transitions to the IG-OFF state, these relatively sudden increases and decreases in voltage value are repeated, whereby the voltage VIN oscillates and the voltage VCC also oscillates. As a result, the voltage for operating the load 4 may become an unstable state.